A current shunt provides for indirect measurement of current values by the measurement of the voltage developed across the shunt by the current passing through the shunt. Typical applications for current shunts include electricity usage control, over-current protection and metering of electricity consumption and generation. In use a shunt of known resistance is provided in series with a load and the voltage developed across the shunt by the load drawn current is measured. The current passing through the shunt is then determined on the basis of Ohm's Law in view of the measured voltage and the known resistance of the shunt.
Certain applications, such as metering of electricity consumption and generation, require measurement to high accuracy over extended periods of time. For example in North America the ANSI C12.20 standard specifies an accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% for Class 0.2 consumption meters. Standards applicable in Europe and elsewhere, such as IEC 62053, specify similar accuracy requirements. It can therefore be appreciated that the resistance of the shunt must be known to high precision to enable a meter to meet regulated accuracy requirements. Although the shunt resistance is normally low to minimise power dissipation and undesirable circuit effects, the shunt is nevertheless liable to heating with temperature drift giving rise to a change in resistance which may cause a loss of measurement accuracy in a shunt of ordinary temperature coefficient of resistance. Shunt resistors formed from managing alloy are therefore widely used in view of their very low temperature coefficient of resistance. It may also be apparent that accurate current measurement depends on measurement of the voltage developed across the shunt being accurate and stable with temperature and lifetime. This is because a change in the transfer gain or lack of precision in references used in the voltage measurement circuit will cause an error. It is normal for these reasons to perform a one-off factory calibration when the shunt and the readout electronics are combined so that a factor related to the actual combined transfer function for current to measurement value, which is determined largely by the shunt resistor and voltage measurement, can be stored and used in subsequent measurements to achieve the desired precision.
An alternative known approach to measuring high values of current involves the use of a current transformer wound on a core, which is disposed around a conductor carrying current to be measured. The current transformer has the advantages over the shunt resistor of being less invasive and providing for isolation from the current carrying conductor. However the current transformer is capable of measuring AC current only. The current transformer generates a current in the secondary coil, which is a ratio of the current in the primary conductor, and the secondary coil current is then turned into a voltage by a load, known as a burden resistor. Accurate measurement of the voltage across the burden resistor combined with accurate knowledge of the transfer function of the primary current to voltage across the burden resistor (i.e. combining the effect of number of turns, the magnetic and the burden resistor) are needed to accurately and precisely measure the current. As with the current shunt, one-off factory calibration is often performed to compensate for inaccuracies in some or all of the elements that contribute to the overall transfer function of primary current to measurement value.
In contrast another approach uses a Hall current probe which is capable of measuring both AC and DC. In an open loop configuration the Hall current probe is, however, liable to non-linearity and temperature drift. In a closed loop configuration the Hall current probe provides an improvement with regards to non-linearity and temperature drift although the weight and size of the configuration increases significantly where higher currents are measured. It is further known to use the Rogowski coil current probe to measure high levels of current. Most known approaches to current measurement, such as by way of the shunt resistor, the current transformer, the Rogowski coil and the Hall current probe, are described and discussed in Current Sensing Techniques: A Review, Silvio Ziegler Robert C. Woodward and Herbert Ho-Ching Iu, IEEE Sensors Journal, Vol. 9, No. 4, April 2009. The different known approaches have their respective advantages and disadvantages.
Load current measurement is often made in conjunction with line voltage measurement, which involves measuring the voltage between the conductors over which the current is delivered, in order to determine the electrical power. As the line voltage is often many times larger than the largest signal that can be safely or conveniently measured, the most common means of measuring the line voltage is through a resistive potential divider between the conductors, which lowers the voltage to be measured by the factor of the divider ratio. The divider ratio and the accuracy of the subsequent voltage measurement chain needs to be known and be sufficiently stable to meet the accuracy requirements of the power measurement application; accuracy requirements for power measurement are specified in the aforementioned standards. Accurate line voltage measurement normally depends on the use of components with good temperature coefficients and known values and on factory calibration, amongst other techniques.
High accuracy power calculation also requires accurate and stable relative phase and frequency response of the load current and the line voltage measurements in order to accurately determine such metrics and differences between active and reactive power, power factor and harmonic content amongst others.
The present inventor has become appreciative of the various shortcomings of known approaches to current measurement and power measurement, such as the approaches described in outline in the preceding paragraphs. It is therefore an object for the present invention to provide improved current measurement apparatus which is configured to provide for accurate measurement of current, for example in a circuit carrying mains current. It is another object for the present invention to provide an improved method of measuring current which provides for accurate measurement of current, for example in a circuit carrying mains current. It is a further object for the present invention to provide improved voltage measurement apparatus which is configured to provide for accurate measurement of line voltage, such as in a circuit carrying mains current, whereby accurate power measurement may be achieved. It is a yet further object for the present invention to provide an improved method of measuring voltage which provides for accurate measurement of line voltage, such as in a circuit carrying mains current, whereby accurate power measurement may be achieved.